Co-channel congestion method and apparatus

ABSTRACT

A system and method for Access Point (AP) channel selection based upon a Channel Quality Index (CQI) is described. The Channel Quality Index (CQI) is a value which quantifies a transmission quality of a channel. The transmission quality is evaluated based on a combination of different types of measured interference in the channel. In one embodiment the different types of measured interference include co-channel congestion, adjacent channel interference and in-band interference. The CQI is a value derived from the measurements, and for example may be a sum of all of the measurements. Each AP of the present invention determines the CQI of potential transmission channels, and selects a channel for use which has the ‘best’ CQI; for example if the CQI is a sum of all measured interferences, the ‘best’ AP is the one with the lowest CQI

FIELD OF THE INVENTION

This invention is generally related to wireless communications, and moreparticularly to a method and apparatus for quantifying transmissionchannel quality in a wireless network.

BACKGROUND OF THE INVENTION

As it is known in the art, a Wireless Local Area Network (WLAN) is alocal-area network that uses high-frequency radio waves, rather thanwires, to communicate between nodes. Various types of wireless LANnetworks exist, and an example of a wireless data network is describedin “IEEE Standard for Information technology—Telecommunications andinformation exchange between systems—Local and metropolitan areanetworks—Specific requirements—Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) specifications, incorporatedherein by reference (hereinafter “802.11”).

Each wireless network typically includes an Access Point device (AP) toallow one or more stations (STAs) to connect to a wired LAN. Accesspoints transmit to the stations by selecting a frequency, or channel, inan available frequency spectrum for communication. 802.11(a) compliantdevices operate in the 5 GHz frequency band using OFDM, with data ratesup to 54 Mbps. 802.11(b) compliant devices operate in the 2.4 GHzfrequency band using direct sequence spread spectrum, with data rates upto 11 Mbps. 802.11(b) devices currently represent the majority ofinstalled wireless LANs. 802.11(g) compliant devices operate in the 2.4GHz frequency band using OFDM, with data rates up to 54 Mbps.

Each AP device in a wireless network selects one channel within adefined frequency band of operation. However there may be interferencein the frequency band generated by competing devices or resulting fromphysical or environmental characteristics at various points in thewireless network. It is desirable for an access point to efficientlyidentify the most desirable transmission channel.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a channel selection methodincludes the steps of calculating a channel quality index (CQI) for aeach one of a plurality of channels available for communication use byan access point in a wireless network, wherein the CQI includes aco-channel congestion measurement for each one of the plurality ofchannels which considers a density of access points in each one of theplurality of channels. The access point selects a preferred channel foroperation according to relative channel quality indices of the pluralityof channels. Such a method permits differentiation between channelshaving similar co-channel congestion noise levels and therebyfacilitates selection of a higher quality channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a local wireless network in which a channelselection method of the present invention may be used;

FIG. 2 is a flow diagram provided to illustrate several steps that maybe performed in the generation of a channel quality index of the presentinvention;

FIG. 3 is a flow diagram provided to illustrate several exemplary stepsthat may be performed in one embodiment of a co-channel congestiondetermination process;

FIGS. 4A and 4B are provided to illustrate the 802.11a Transmit SpectrumMask and the spill over of the 802.11a Transmit Spectrum Mask ofadjacent channels;

FIG. 5 illustrates the 802.11b Transmit Spectrum Mask; and

FIG. 6 is a flow diagram provided to show the use of the Channel QualityIndices by an Access Point during a channel selection process.

DETAILED DESCRIPTION

In accordance with the present invention, a system and method for AccessPoint (AP) channel selection based upon a Channel Quality Index (CQI)will now be shown and described. The Channel Quality Index (CQI) of thepresent invention is a value which quantifies a transmission quality ofa channel. The transmission quality is evaluated based on a combinationof different types of measured interference in the channel. In oneembodiment the different types of measured interference includeco-channel congestion, adjacent channel interference and in-bandinterference, although the present invention is not limited to anyspecific combination of interference considerations. The CQI is a valuederived from the measurements, and for example may be a sum of all ofthe measurements. Each AP of the present invention determines the CQI ofpotential transmission channels, and selects a channel for use which hasthe ‘best’ CQI; for example if the CQI is a sum of all measuredinterferences, the ‘best’ AP is the one with the lowest CQI.

Referring now to FIG. 1, a typical wireless communications environment10 includes access point (AP) devices 12 and 13 that interface between awired communications medium 14 and wireless devices 16-19 to providenetwork access to the wireless devices. Wireless device 16 can thuscommunicate with wired devices 15 a-15 c and other wireless devices16-19 via the access device 12. Similarly, wireless devices 17, 18 and19 can communicate with wired devices 15 a-15 c, wireless device 16 andwith each other via access device 13. These access devices 12 and 13 arereferred to by various names depending upon the wireless architectureemployed, and are herein referred to as “access points” or “APs”. Thewireless devices 16-19 also have various architecture dependent namesand are herein referred to as “stations” or STAs. A wirelesscommunications capable device may be an AP, or a STA, or both.

In a wireless network such as network 10, each AP communicates with thestations over transmission channels, wherein channel numbers are shownin brackets in FIG. 1. and indicate a designated radio frequencyavailable for use by the transmitter and receiver of two communicatingdevices. Thus AP 12 communicates with STA 16 via channel (1), and AP 13communicates with stations 17, 18 and 19 over channel (2). Each APtypically selects a preferred channel for communication atinitialization. In addition, as described in pending application“(Attorney Docket Number 160-091), entitled Backup Channel Selection inWLANs, filed on Apr. 11, 2005 by Yuen et al, and incorporated herein byreference, an AP may also periodically monitor unused channels forevaluation purposes. In the event that the AP identifies that analternate channel has preferred communication characteristics over acurrently used channel, the AP may transition communications to thepreferred alternate channel. The Channel Quality Index of the presentinvention may be a value that is stored by the APs to expedite andimprove the quality of channel selection decisions. FIG. 1 illustratesthat each AP 12 and 13 of the present invention includes a respectiveCQI table 25, 27.

FIG. 2 is a flow diagram illustrating several exemplary steps that maybe taken to measure different interferences, which measurements may thenbe used to calculate a CQI for a given channel. Although the steps areshown and will be described in a certain sequence, it should beunderstood that they may be performed simultaneously or in any order. Inaddition, each measurement may be updated at different rates, and thusthe present invention is not to be limited by the sequence in which themeasurements are ascertained, or to any particular order by which thesteps are described below.

1. Co-Channel Congestion:

The co-channel congestion (CCC) is congestion resulting from thesimultaneous transmission of APs of a compatible transceiver type in thesame channel. The co-channel congestion (CCC) measurement 102 is made interms of measured noise power in the wireless medium or Ether basedchannel; in this embodiment CCC will be measured in decibels permilliwatt (dBms). If the channel is empty, the co-channel congestionvalue is the platform noise floor value for the channel. Platform NoiseFloor is a configuration parameter for each radio. For example, a goodWLAN reference might be −95 dBm. If the channel is not empty, and thereis more than one AP, in one embodiment the AP with the highest power onthe channel is used for the measurement. Thus the Co-channel congestion(CCC) value is determined according to Equation I below.Co-channel congestion=MAX(Platform Noise Floor Value, Loudest AP on thechannel)  Equation I:

where the MAX function selects the largest of either the Platform Noiseor the Loudest AP.

As shown at step 160 of FIG. 2, in one embodiment, a user selectedweight value w₁ may be selectively applied to the co-channel congestionpredication value to increase or decrease the impact of co-channelcongestion when determining the overall channel quality index (CQI).

The above embodiment for determining a co-channel congestion valueeffectively measures the loudest AP on the channel. This methodadvantageously quantifies co-channel congestion in an expedient andstraightforward manner. In an alternative embodiment of this invention,information regarding the density of each channel (i.e. the number ofAPs on the channel) is considered when calculating the co-channelcongestion. One advantage of including channel density effects whencalculating the CCC is that more accurately represents the activity inthe channel and thus improve the quality of the CQI calculation. Forexample, when looking only at the loudest AP on a channel using EquationI above, a channel with one AP of −40 dBm would have the same CCC valueas a channel in which 3 APs are being received at signal powers of −40dBm, −47, and −54.

An improved CCC value can be obtained by incorporating informationregarding the density of APs in the channel. Exemplary steps that may betaken in the alternate process of calculating CCC are shown in FIG. 3.At step 150, the noise value for the loudest AP on the channel ismeasured as described in Equation I. At step 152, the contributions ofother APs occupying the same channel are determined by measuring thesignal strength of each of the APs, converting the signal strength intoa Contribution Factor (f) for the AP, and adding a Contribution Factor(f) to the CCC obtained at step 150. The Contribution Factor (f) is avalue associated with a range of signals strengths. The particularContribution Factor (f) associated with a signal strength range is amatter of design choice, but is generally selected to increase ordecrease in accordance with the signal strength of the associated AP.The maximum contribution factor is selected according to a desiredmaximum density of a channel, and should be high enough to reflect theexistence of another AP, but low enough so that use of the channel wouldstill be encouraged for lower AP densities. In addition, the values areselected so that the contribution factor decreases along with the signalstrength, so that less weight is given to APs that are a fartherdistance from the loudest. AP in the channel. It is noted that althougha linear relationship between the signal power levels and the congestionfactor is shown in the tables, non-linear relationships which moreheavily penalize closer APs are also envisioned, and it should be notedthat congestion factors shown are exemplary only. Table I belowillustrates exemplary Contribution Factors for A band communications,and table II below illustrates exemplary Contribution Factors for B/Gband communications: TABLE I Received Power dBs to add >−45 dBm 6 −46dBm to −51 dBm 5 −52 dBm to −57 dBm 4 −58 dBm to −63 dBm 3 −64 dBm to−69 dBm 2 −70 dBm or less 1

TABLE II Received Power dBs to add >−38 dBm 6 −39 dBm to −44 dBm 5 −45dBm to −50 dBm 4 −51 dBm to −56 dBm 3 −57 dBm to −62 dBm 2 −63 dBm orless 1

Once the Contribution Factors (f) for each other AP in the channel iscalculated, at step 154 the co-channel congestion can be calculated byadding the Signal Strength (in dBs) of the loudest AP in the channelwith the Contribution Factors of each of the other APs in the channel.Thus, returning again to the above example comparing a channel with oneAP at −40 dBm to a channel with three APs, having respective signalstrengths of −40 dBm, −47 dBm and −54 dBm, the co-channel congestion forthis channel would be (assuming we are using the B/G band) is:Co-channel congestion=−40 dBm+f(−47 dBm)+f(−54 dBm)Co-channel congestion=−40 dBm+4 dB+3 dB=−33 dBm

The resulting calculation thus indicates that the channel with three APsis of lower quality than the channel with one AP, by a 7 dBs margin,even though both channels have the same in-band noise level.

The co-channel congestion measurement may be further adapted in responseto other network considerations. For example, if it is desirable toavoid other WLANs, signal levels of all APs heard in the channel aretaken into consideration. However, if it is not necessary to avoid otherWLAN, it may be desirable to consider only the APs with SSIDs indicatingthat they are in the network under evaluation. Other methods forselecting APs to consider, for example based on ranges or signalstrengths, may also be used, and the present invention is not limitedmerely by the specific embodiment or example provided above.

2. Adjacent Channel Interference:

Adjacent Channel Interference (ACI) is interference caused by extraneouspower from a signal in an adjacent channel. Adjacent channelinterference is caused by imperfect filtering, such as incompletefiltering of unwanted modulation products in frequency modulation (FM)systems, improper tuning, and/or poor frequency control in thetransmitting reference channel along with the receiving radiocharacteristics for the interfering channel.

ACI includes a predictable amount of adjacent channel interference whichmay be added to the quietness of the channel obtained from the CCCmeasurement. Signals generated by commercially available wirelessequipment tend to generate some amount of energy outside of theirapproved spectrum band. This is called side band emissions. This also istrue of other wireless devices, such as Bluetooth, cordless telephonesand devices. Although filtering is usually done to minimize RFinterference from adjacent channels, some of the energy spills into theadjacent channel and causes interference with products operating on theadjacent channel: If the ACI is much stronger than the 802.11 signal,side band energy from the ACI can dominate the channel's noise floor.For example, FIG. 4A illustrates the 802.11a transmit spectrum mask. Inorder to more fully populate the frequency spectrum with channels, thechannels are typically closely spaced within the spectrum. Such spacing,however, results in a certain amount of spillover of side lobe energybetween channels, as shown in FIG. 4B.

For example, in FIG. 4A, at 20 MHz from the center frequency on eachside of the 802.11a transmit spectrum mask, the adjacent channelrejection power level is −28 dB. Since the channel separation for802.11a is 20 MHz, we can expect the adjacent channel rejection powerlevel to be at least 28 dB. However, because signals are transmitted inOFDM, an additional 18 dB is suppressed, resulting in a 46 dB adjacentchannel rejection power level for 802.11a APs that are one channel away.FIG. 5 illustrates ACI caused by side lobes in the 802.11b transmitspectrum mask.

Note every mode of operation uses channels having different spacing, andthus the number of adjacent channels that are used in calculating theACI at step 120 (FIG. 2) is determined by a mode of operation of the AP.For example, for APs operating in 802.11a mode, calculations areperformed as though each AP had to consider the effects only of oneneighboring channel on each side. For other modes of operation, forexample for those platforms that support Turbo channels, multiple sidechannels are incorporated into the ACI measurement calculation. Notethat the present invention is not limited to the inclusion of anyspecific number of side channels in the ACI measurement. Thus, the ACImeasurement can be determined through the use of Equation II below:P _(ACI)=10 log Σ[10ˆ( y _(i)/10)];  Equation II:

where P_(ACI) is the predicted adjacent channel interference and they_(i) is the adjacent channel rejection noise level illustrated in FIGS.4A and 5.

The adjacent channel interference raises the average channel receivenoise level and in turn linearly reduces the RSSI value of the receivedsignal. To represent this effect, the predicted ACI is added to theco-channel congestion for each channel. For example if the adjacentchannel interference increased the average receive noise level by 3 dBs,3 dBs is to the AP power measured on the channel to determine channelquality. When comparing two channels with APs having the same signalstrength, but one channel has adjacent channel interference and otherdoesn't, the channel that has no interference is favored.

Using the above theory, the ACI for each 802.11a channel can begenerally calculated using the equations in below Table III (for 20 mHzspaced channels): TABLE III Amount of Spill-Over from 802.11a AdjacentOperating Channels Predicted Adjacent Channel Spill-Over Number ofChannels Away y_(i) 1 X₁+ (−46 dB) 2 X₂+ (−58 dB) 3 X₃+ (−62 dB) 4 X₄+(−66 dB)(Notes: x_(i) = signal strength of the adjacent channel AP in dBmmeasured on the channel they are on; If yi is smaller than the platformnoise floor, we can ignore it)

and the ACI for each 802.11b channel can be generally calculated usingthe equations in below Table IV (for 5 mHz spaced channels): TABLE IVAmount of Spill-Over from 802.11b/g Adjacent Operating ChannelsPredicted Adjacent Channel Spill-Over Number of Channels Away y_(i) 1X₁+ (−0 dB)  2 X₂+ (−6 dB)  3 X₃+ (−32 dB) 4 X₄+ (−46 dB)(Notes: x_(i) = signal strength of the adjacent channel AP in dBmmeasured on the channel they are on; If yi is smaller than the platformnoise floor, we can ignore it)

As shown at step 160 of FIG. 2, in one embodiment, a user selectedweight value w₂ may be selectively applied to the adjacent channelinterference predication value to increase or decrease the impact ofadjacent channel noise when determining the overall channel qualityindex (CQI).

3). In Band Noise

The in-band noise power ratio is the ratio of (a) the mean noise powermeasured in any channel, with all channels loaded with white noise, to(b) the mean noise power measured in the same channel, with all channelsbut the measured channel loaded with white noise. According to thepresent invention, the in-band noise power level correlates to thereceive power level of the hardware, and the value is measured for eachusable channel by an AP at step 140 of FIG. 2. The in-band noise of eachchannel can be measured after the channel is scanned for APs. In oneembodiment, the receive power level of the hardware is monitored onceper scan cycle to average out slow pulsing noise, although this is not arequirement. Any deviation of the receive power level from the platformnoise floor is an indication of the amount of in-band noise, or externalnoise added to the channel. For example if the measured receive powerlevel for the 11 g radio is −90 dBm, then the added noise is 4 dB.

According to one embodiment of the invention, if the receive power levelfor a channel is determined to cross a pre-determined upper bound powerlevel, the channel as identified as unusable and removed from furtherpower-up channel selection consideration. Upper bounds may be dictatedby the hardware of the AP.

To compensate for possible small variations of measured receive powerlevels for the APs, in one embodiment, in-band noise is quantized inorder to provide incremental steps for in-band noise indication. Theplatform noise floor of the system is compared against the in-band addednoise floor. The amount by which the added noise floor exceeds theplatform noise floor is used to determine the weighted noise addition,as shown in Table V below. An equation that may be used in calculatingin-band noise for 802.11a operating mode is provided in below EquationIII, where y is the platform noise floor:

Total In-Band Interference Power:P _(IB) =y−(−91 dBm); if y<−91 dBm, y=−91 dBm  Equation III:

-   -   An equation that may be used in calculating in-band noise for        802.11b/g operating mode is provided in below Equation IV,        wherein y is the platform noise floor:

Total In-Band Interference Power:P _(IB) =y−(−94 dBm); if y<−94 dBm, y=−94 dBm  Equation IV:

As it is known, the presence of noise can reduce the capacity of aninformation channel. The relationship between the quantity of noise andthe reduction of the capacity of the channel is non-linear. According toone aspect of the invention, to account for the reducing in capacity, anoise degradation factor is applied to the measured added noise value.Table V illustrates the non-linear relationship between increasing noiseand signal degradation, showing that the degradation factors are notlinear as the in-band noise value increases. Table V is indexed by theP_(IB) obtained above in Equation III or Equation IV the in-band noiseincreases, the CQI is dominated by the noise over the co-channelcongestion. It should be noted that the weights applied to the in-bandnoise value are merely exemplary; other methods of assigning weightvalues to the in-band noise measurements may alternatively be usedwithout impacting the scope of the present invention. TABLE V Measuredin-band added noise Weighted in-band above the platform noise floornoise addition 1 2.1 2 3.1 3 4.3 4 5.3 5 6.6 6 7.6 7 9 8 10 9 11.7 1012.7 11 14.2 12 15.2 13 17 14 18 15 19.6 16 21.3 17 22.3 18 24.4 19 25.420 28 21 30.1 22 31.8 23 34.9 24 40 25 45 26 51 27 59 28 73.7 29 82.3The weight is dB for dB above 29

In addition, as shown at step 160 of FIG. 2, in one embodiment, a userselected weight value w₃ may be selectively applied to the in-bandinterference predication value to increase or decrease the impact ofin-band interference when determining the overall channel quality index(CQI).

Channel Quality Index Calculation

According to one embodiment, the Channel Quality Index (CQI) is the sumof all three interference measurements or predictions for each channel.The channel with the lowest CQI is the channel having the lowestinterference, and thereby is determined to be the channel which is thepreferred channel for transmissions.

Table VI below illustrates exemplary values that may be measured foreach one of eleven bands. Given the 5 mHz spacing of the channels, theAP may use either channel 1, channel 6 or channel 11. In the belowexample, channel 1 will be picked even though channel 6 is empty,because the adjacent channel interference of channel 5 reduces thequality of the channel. TABLE VI Adjacent Channel Power Channelincluding Power in Interference adjacent In-Band Channel dBm Powerchannels Noise CQI 1 −36 −74.93 −36.00 4 −32.00 2 −94(empty) 3−94(empty) 4 −94(empty) 5 −29 6 −94(empty) −28.99 −28.99 0 −28.99 7−94(empty) 8 −48 9 −94(empty) 10 −94(empty) 11 −29 −79.79 −29.00 2−27.00

Referring briefly to FIG. 6, a flow diagram illustrating how the CQI ofthe present invention may be used for channel selection is shown. Atstep 200, an AP scans each available transmission channel to identifydifferent interferences and generates a CQI for that channel asdiscussed in the two embodiments described in FIGS. 2 and 3. At step202, the AP selects a highest quality channel; that channel being theone with the lowest interference value. Once the channel is selected, atstep 203 the AP begins issuing communications on the channel.Concurrently, the AP continues to evaluate interference on the alternatechannels, updating the CQI values for each channel as the networkcharacteristics change. At some point the AP may decide to changechannels. The AP may decide to change channels for any variety ofreasons, including because interference on the current channel hasdegraded communications. The AP may decide to switch channels when abetter alternate channel is discovered, although it is recognized thatswitching between channels will temporarily degrade wireless networkperformance. Suffice it to say that when the AP decides to changechannels, the CQI values of the channels found in step 204 are comparedat step 202 again to select the one with lowest CQI, and thus the lowestamount of interference.

Accordingly a system, method and apparatus for identifying andquantifying a channel quality index to permit an access point to morequickly identify a preferable channel for communication has been shownand described. Having described an exemplary embodiment of the presentinvention, it will be appreciated that various modifications may be madewithout diverging from the spirit and scope of the invention. The abovedescription has talked of the present invention in terms of functionalblocks delineated in a manner to facilitate description. However, itshould be noted that the invention may be implemented in a variety ofarrangements, using hardware, software or a combination thereof, and thepresent invention is not limited to the disclosed embodiment. Forexample, FIG. 6 is a flowchart illustration of methods, apparatus(systems) and computer program products according to an embodiment ofthe invention. It will be understood that each block of the flowchartillustrations, and combinations of blocks in the flowchartillustrations, can be implemented by computer program instructions.These computer program instructions may be loaded onto a computer orother programmable data processing apparatus to produce a machine, suchthat the instructions which execute on the computer or otherprogrammable data processing apparatus create means for implementing thefunctions specified in the flowchart block or blocks. These computerprogram instructions may also be stored in a computer-readable memorythat can direct a computer or other programmable data processingapparatus to function in a particular manner, such that the instructionsstored in the computer-readable memory produce an article of manufactureincluding instruction means which implement the function specified inthe flowchart block or blocks. The computer program instructions mayalso be loaded onto a computer or other programmable data processingapparatus to cause a series of operational steps to be performed on thecomputer or other programmable apparatus to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide steps for implementingthe functions specified in the flowchart block or blocks.

Those skilled in the art should readily appreciate that programsdefining the functions represented by functional blocks of FIGS. 1-6 ofthe present invention can be delivered to a computer in many forms;including, but not limited to: (a) information permanently stored onnon-writable storage media (e.g. read only memory devices within acomputer such as ROM or CD-ROM disks readable by a computer I/Oattachment); (b) information alterably stored on writable storage media(e.g. floppy disks and hard drives); or (c) information conveyed to acomputer through communication media for example using basebandsignaling or broadband signaling techniques, including carrier wavesignaling techniques, such as over computer or telephone networks via amodem.

While the invention is described through the above exemplaryembodiments, it will be understood by those of ordinary skill in the artthat modification to and variation of the illustrated embodiments may bemade without departing from the inventive concepts herein disclosed.Accordingly, the invention should not be viewed as limited except by thescope and spirit of the appended claims.

1. A channel selection method includes the steps of: calculating achannel quality index (CQI) for a each one of a plurality of channelsavailable for communication use by an access point in a wirelessnetwork, wherein each CQI includes a co-channel congestion measurementwhich considers a density of access points in the respective channel;and the access point selecting a preferred channel for operationaccording to relative channel quality indices of the plurality ofchannels.
 2. The channel selection method of claim 1 wherein theco-channel congestion measurement for each channel is obtained by:measuring a signal power of each of a plurality of access points in thechannel; selecting a signal power from a loudest access point having themaximum signal power; for each of the plurality of access points thatare not the loudest access point, associating a congestion factor withthe measured signal power of the respective access point; and computingthe co-channel congestion measurement based on the selected signal powerand the plurality of congestion factors.
 3. The method of claim 2,wherein a value of the congestion factor is related to the signal powerof the access point.
 4. The method according to claim 3, wherein therelationship between the congestion factor and the signal power islinear.
 5. The method according to claim 3, wherein the relationshipbetween the congestion factor and the signal power is non-linear.